This invention relates to a time division cellular mobile communications system in which the radio interface between a mobile station and a base station consists of traffic frames comprising of several time slots in which the mobile stations can transmit in one time slot of an uplink frame to the network a request in an access burst to be provided with a radio connection for traffic.
A radio system based on time division multiple access (TDMA) features frame structures so that the transmission and reception frames consist of time slots. The transmission occurs in a certain frequency and in a certain time slot, and the reception occurs in a certain frequency and in a certain time slot. The transmission and reception frequencies can be the same frequency, as in, for example, a DECT system, or they can be different frequencies, as in, for example, GSM and DCS systems, which are thereby both time division and frequency division systems. As an example of a TDMA system the following examines a GSM/DCS system. In this system the length of one TDMA frame is 4.615 ms and it consists of eight time slots, numbered from 0 to 7. The number of a time slot is referred to by the abbreviation TN (Time Slot Number). The duration of one time slot has been defined as (576+12/13) xcexcs or the duration of (156+1/4) bits. A full speed traffic channel TCH consists of every eighth time slot in a cyclic manner, so from the viewpoint of the network one carrier can be used to form eight traffic channels. The traffic from the mobile station to the base station (uplink direction) and the traffic from the base station to the mobile station (downlink direction) has been arranged in such a manner that the reception in the base station occurs three burst durations later than the transmission. In this case the transmission time slot number of a transmission frame TN and the time slot number of a reception frame (TN) are identical. This is illustrated in FIG. 1 the upper part of which shows the consecutive time slots of a certain transmission carrier and the lower part the consecutive time slots of the reception frequency related to this carrier. The frequencies are generated by using a transceiver TRX. A TRX unit can change its frequency, in which case a different frequency can be in use during each time slot. There must be several TRX units so that it is possible to use several frequencies within one time slot.
The consecutive time slots TN=0 of reception frames of a certain frequency in a base station form the RACH (Random Access Channel) and in this channel the network receives the requests transmitted by mobile stations to be given access to channel resources. The RACH is an uplink channel only. If the request is accepted, the network transmits on the PAGCH (Paging and Access Grant Channel) the acknowledgement of the request and the information as to which channel the mobile station must switch to. The PAGCH is a downlink channel only and it consists of the consecutive time intervals TN=0 of the frames of a certain transmission frequency.
The following bursts can be distinguished from one another: access burst, F and S bursts and normal burst. The difference between them is in their time-amplitude profile.
The normal burst is the longest burst of all; its duration is 148 bits and it is used in the traffic channel and in most signalling cases. It includes two sequences of 58 bits which are separated by an training sequence of 26 bits, and, at the beginning and the end of the burst there are three tailing bits. The duration of normal bursts must be slightly smaller than that of a time slot so that when a base station receives, the bursts transmitted in adjoining time slots do not overlap. The transmission of the normal burst in a mobile station starts by the amount of the timing advance TA before the reception time slot in the base station begins, in which case the burst arrives in the time slot right at the beginning of the slot and the entire burst fits in the slot. F and S bursts are only transmitted in the downlink direction in frequency correction and synchronization channels and they are used when the mobile stations synchronize themselves to the base station and to correct frequency errors caused by movement.
The access burst is only used in the uplink direction at the beginning of a connection when the propagation delay between the base station and mobile station is not known. This is the situation when the mobile station contacts the network via the RACH and in some cases in handover situations when the mobile station moves to a new cell. The access burst includes an training sequence of 41 bits, 36 information bits and 7 tailing bits at the beginning and 3 tailing bits at the end, or a total of 87 bits (the length of a normal burst is 148 bits). The access burst is thereby very short and no other bursts are used in the RACH. The base station receives access bursts in the RACH, in other words, in the time slot TN=0 and, if the network simultaneously receives several bursts, it rejects them all. The mobile station retransmits the access burst until the request is accepted and a traffic channel is assigned to the mobile station. The training sequence of the burst is longer than that of a normal burst so that the probability of success in the demodulation of the burst is high. This is important because the receiver does not know the level of the burst, frequency error, or time of arrival within the time slot. Because the propagation delay between the mobile station and base station is not known when the access burst is used, the arrival of the access burst to the base station features a time error compared to the reception time slot, the length of which is two times the length of the propagation delay. To compensate this the duration of the access burst is short so the mobile station can progress as far as 35 km before the access burst misses the reception time slot.
The aforementioned 35 km is at the same time in theory the maximum cell radius in the network. When it is desirable to expand the system to sparsely populated or uninhabited regions by arranging the radio coverage of the system to cover at least the main roads, complete radio coverage can only be achieved by placing fully equipped base stations at intervals of 70 kilometers. This is a rather expensive solution because a fully equipped base station contains a great deal of expensive components and the base station link mast must be extremely high. However, the distance can be increased, especially on flat terrain, by building the base station masts even higher and by using only every other time slot of a frame. The time slots used are the even ones, because the time slot TN=0 is reserved for access bursts. When only every other time slot is used, the timing advance values achieved are tremendous, so the cell radius can be expanded to a much higher value than 35 km, at the expense of channel efficiency.
On the other hand, when it is desirable to have good radio coverage in a densely populated region, fully equipped base stations must be placed very close to one another. This must be done, of course, because of the great traffic density, but especially when it is desirable to arrange coverage in indoor spaces, such as car parks, department stores, subway stations, etc., in other words, places where there are a lot of people but where radio wave penetration is poor. More base stations must also be established if it is desirable to arrange radio coverage to shadow regions between and behind tall terrain features. When the base stations are added, the number of Abis interfaces also increases which include the interfaces between the base station controller BSC and the base stations controlled by it.
The patent application FI-933091 describes a method to expand the cell radius to a value considerably in excess of 35 km. The application suggests that the timing of the receiver of one transceiver unit is delayed in relation to the transmitter. This is implemented by delaying the frame clock and time slot clock of the receiver. In this case, the burst that arrives from a distance of over 35 km hits the delayed reception time slot. Bursts that arrive from distances of under 35 km do not hit this time slot but use the time slot with normal timing. The unit of the delayed receiver thereby serves a circular region around the actual basic cell. In this manner it is possible to increase a cell with radius of 35 km to a cell with radius of 60 km.
Expansion of the system and to achieve perfect radio coverage in indoor and outdoor shadow regions both require more fully equipped base stations which significantly increases the costs. Because of this the possibility of using repeater stations as base stations has been studied. The idea of a repeater station has been known for a long time, for example, from broadcasting, and it is used in the analog cellular system to some extent. It is characteristic of these solutions that the repeater station merely amplifies the incoming signal and retransmits it on the same frequency. Application of this method is well suited to AMPS and other frequency division duplex (FDD) systems, because the envelope of the received signal corresponds with the envelope of a Rayleigh faded signal. However, so far it has not been possible to apply the idea to time division TDMA systems or in FDD/TDD systems primarily because of the time division nature of the systems, in which case the time dispersion (frequency selectivity) must be taken into account.
Firstly, the received signal is no longer a single Rayleigh faded signal but, because of multipath propagation, the sum of several Rayleigh-fading signals with different delay. The channel equalizers of the receivers in base stations and mobile stations and the bit pattern, or training sequence, are such that the system can correct the received signal up to a delay of 16 xcexcs, but if a repeater station repeats, in a time division multiplex system, the frequencies of a base station as they are, the base station can no longer distinguish whether a signal was intended for the base station or the repeater, and it can no longer, in general, distinguish from one another the signals which arrive on the same frequency.
Secondly, from the viewpoint of the network, the base station and the repeater station linked to it form one cell and the network does not get any information about whether the mobile station is located within the area of the base station or the area of the repeater station linked to the base station. The current time division cellular networks do not contain elements by using of which the network is able to detect whether the radio path contains extra elements, such as repeaters. This is a significant drawback especially if it is desirable to have individual tariffs. The operator may, for example, want to compensate the costs caused by the coverage arrangements of shadow regions by charging more for calls made in shadow regions.
The objective of this invention is thereby a base station arrangement which is suitable for a time division multiplex cellular system which makes it possible to use both an extended cell and to cover the shadow areas without adding the interfaces in a base station and between base stations and, additionally, makes it possible to detect in the area of which repeater station a call has been made.
The set objectives are achieved by using the specifications expressed in independent claims.
All connections from the area of a repeater pass to the base station controller through the same base station. It is possible that the connections of even several repeaters pass through the base station, in which case the repeater group formed by the repeaters can cover the shadow regions of the cell or the repeater can be used to extend the radio coverage of the base station to cover a larger area, in other words, to form a so-called extended cell. The base station operates on the frequencies assigned to it in network planning, but each repeater performs a frequency conversion so that during a traffic connection the mobile stations operate on different transmission/reception frequencies to the direction of the repeater compared to those which are used in traffic between the repeater and the base station. Because the frequencies are different, the transmission signal of the mobile station does not disturb the operation of the base station by causing co-channel interference.
The use of different frequencies makes it possible to use frequency hopping within the repeater area. In this case, the base station jumps in a normal manner from the frequency assigned to it in network planning to another at the same time as the mobile station jumps from one frequency to another within the frequency band used by the repeater.
The use of different frequencies also makes it possible to use a handover function within the repeater. When the mobile station moves to an area where it hears the new signal frequency better than the one on which it is operating, it signals the base station controller of the fact. The base station controller commands the mobile station to switch to the new frequency but continues its own operation on the old frequency, in other words, the frequency between the base station and the repeater does not change.
When the repeater is located within the cell, there is no need to alter the timings, but the timing advance value TA indicated to the mobile station by the network is valid as it is. When the repeater is located at the cell boundary and its coverage area extends the cell, the situation requires a timing advance greater than possible in the system as it is. In this case, it is possible to use a method according to the patent application FI-993091 and delay the reception in the base station. It is also possible to implement a solution in which there is no one-to-one relation between the reception time slot of the base station and the transmission time slot of the mobile station to the direction of the repeater. The same is also valid for time slots in the downlink direction of the connection. In this case, the repeater must contain buffers, in which case the repeater first receives the burst transmitted by the mobile station and relays the burst after a delay in the reception time slot of the base station. This is, however, a difficult method.
According to another characteristic feature of the invention the repeater synchronizes itself to the transmission from the base station and determines from it the time slots used for different purposes. In this manner the repeater detects the time slots for uplink direction RACH, in which case it can observe whether there are any access bursts arriving in the time slot from the repeater area. When the repeater receives a burst, it also transmits an extra burst which contains an individual identifier of the repeater in the same time slot in which it transmits this mobile station access burst to the base station. The identifier can be a code similar to the training sequence. The base station controller decodes the identifier, in which case it is possible to identify the repeater via which the mobile station has started the connection. The base station controller knows the channel states, so it allocates a free traffic channel to the base station controller and commands the mobile station to a traffic channel which is not the same as the base station channel, but one of the channels used by the repeater. The base station does not know this but operates in the usual manner.
According to the preferable embodiment, the efficiency of the resource use of the repeater group can be improved so that the handover between the repeaters is made possible so that the traffic time slot used by the base station remains the same as before the handover, in other words, from the viewpoint of the base station the handover is performed to the same time slot. In this case, it is possible to switch from one repeater to another within a repeater group, even if the base station did not have any free time slots.